Перед отправкой я решила скопировать важный текст в заметки и сделать ещё одну резервную копию.

Because the preposition перед meaning before normally takes the instrumental case.

• перед отправкой = before sending / before the sending
• Base noun: отправка = sending
• Instrumental singular: отправкой

This is a very common pattern:

• перед уроком = before the lesson
• перед встречей = before the meeting
• перед поездкой = before the trip

So перед отправкой literally means before the sending, but in natural English it is usually before sending.

Russian often uses a noun phrase where English prefers a clause.

So instead of saying something like before I send it, Russian can say:

• перед отправкой = before sending / before the send

This sounds natural in Russian and is very common in more neutral or written-style language.

A more explicitly verbal version is also possible in Russian, for example:

• перед тем как отправить... = before sending... / before I send...

So the sentence could have been phrased differently, but перед отправкой is perfectly normal and concise.

Because the speaker is female.

In the past tense, Russian verbs agree with the gender of the subject in the singular:

• я решил = I decided (male speaker)
• я решила = I decided (female speaker)

So решила tells us that the person speaking is a woman.

Because скопировать is perfective, and the sentence is talking about completing a single action.

• копировать = imperfective, more like to copy / to be copying
• скопировать = perfective, to copy successfully / to make a copy

Here the speaker decided to do two complete actions:

• скопировать важный текст
• сделать ещё одну резервную копию

Both are one-time, completed actions, so perfective verbs are natural.

For the same reason as скопировать: сделать is perfective and refers to a completed result.

• делать = to do / to be doing
• сделать = to do, to get done, to complete

In this sentence, the speaker is talking about making an additional backup as a finished action, not about the process in general.

So:

• сделать резервную копию = to make a backup copy

Because it is the direct object of скопировать, so it is in the accusative case.

• важный is masculine singular
• текст is a masculine singular inanimate noun

For masculine inanimate nouns, the accusative looks the same as the nominative:

• nominative: важный текст
• accusative: важный текст

So even though it is the object, the form does not visibly change here.

Compare with an animate masculine noun, where accusative would change:

• я вижу нового студента = I see the new student

В заметки means into Notes / into my notes / into a notes app, depending on context.

The noun заметка usually means a note.
Its plural is заметки = notes.

With в + accusative, Russian often expresses movement into something:

• в папку = into the folder
• в архив = into the archive
• в заметки = into notes

In modern usage, заметки can also refer to a Notes app or a notes section on a device. So here it likely means the speaker copied the text into their notes app or note file.

Ещё одну means one more or another.

• ещё = still / yet / more, depending on context
• одну = one in the feminine accusative singular

Together:

• ещё одну резервную копию = one more backup copy / another backup copy

It implies that there is already at least one copy, and the speaker wants an additional one.

Because копия is a feminine noun.

Russian adjectives and numbers must agree with the noun in gender, number, and case.

Here, копию is feminine accusative singular, so the words modifying it must match:

• одну = feminine accusative singular of one
• резервную = feminine accusative singular of резервный
• копию = accusative singular of копия

So:

• ещё одну резервную копию = another backup copy

Literally it means reserve copy.

In actual usage, it means:

• backup copy
• often simply backup

This is the standard Russian expression for a backup in computing and everyday contexts.

Forms here:

• nominative: резервная копия
• accusative: резервную копию

Because решить can be followed by an infinitive meaning to decide to do something.

So:

• я решила скопировать... = I decided to copy...
• и сделать... = and to make...

Russian often uses one finite verb plus several coordinated infinitives:

• Я хочу поесть и отдохнуть. = I want to eat and rest.
• Она решила позвонить и написать. = She decided to call and write.

That is exactly what is happening here.

Yes, Russian word order is fairly flexible, though the original order sounds natural.

Current version:

• Перед отправкой я решила скопировать важный текст в заметки и сделать ещё одну резервную копию.

This is a neutral, natural order:

1. time expression first: before sending
2. subject + verb: I decided
3. actions: to copy... and make...

You could rearrange parts for emphasis, for example:

• Я решила перед отправкой скопировать важный текст в заметки и сделать ещё одну резервную копию.

This is also correct, but the original sentence feels very normal and smooth.

By itself, отправка means sending / dispatch / submission, depending on context. In this sentence it most naturally refers to sending the thing being discussed.

Because the meaning is already known to the learner, the exact object of sending is left unstated in the Russian sentence. Russian often does this when the context makes it clear.

So перед отправкой can be understood as:

• before sending it
• before submitting it
• before dispatching it

The exact English wording depends on the situation.